Redox catalyst systems are widely used to accelerate polymerization of unsaturated compounds in a variety of applications. Redox catalyst systems generally include an oxidizing agent and a reducing agent (also referred to as accelerators) which are co-reactive at room temperature to generate free radicals to initiate addition polymerization reactions. In an exemplary redox system, the oxidizing agent is an organic peroxide and the reducing agent is a tertiary amine. Exemplary tertiary amines which are commercially available include N,N-diethanol-p-toluidine, N,N-dimethylaniline, and N,N-dimethyl-p-toluidine.
A common problem associated with polymerization of unsaturated compounds, such as acrylic adhesives, is referred to as "air inhibition." Atmospheric oxygen is a powerful inhibitor of free radical reactions. In many applications, it is a practice to use a slight excess of adhesive to ensure complete coverage of the area to be bonded. This can result in some adhesive being squeezed out around the bond area and thus exposed to air, resulting in a layer of unreacted monomer remaining on the surface.
This can be problematic for several reasons. The edge of the bond can be weakened, thus reducing bond strength of the assembly as applied stresses are concentrated at the edges of the bond area. The uncured adhesive surface can also allow free monomer to escape into the atmosphere, contributing to the perceived odor of such adhesives. In addition, an uncured surface can result in adhesive being transferred to other parts of the assembly and increasing clean-up costs. These problems can be aggravated if cure occurs during periods of elevated temperature and/or high relative humidity.
U.S. Pat. No. 3,321,351 teaches the addition of small quantities of waxy materials, such as paraffin, montan wax, beeswax, ceresine wax, spermaceti, and the like, to acrylic based adhesive compositions. The addition of these waxy materials can improve the open time of the adhesives (i.e., the length of time the adhesive can be applied to a surface), as well as provide some improvements in surface cure. Although the use of such waxy materials can provide advantages in surface cure, air inhibition for acrylic adhesives remains a problem.
U.S. Pat. No. 4,421,879 to Eimers, et al. is directed to the use of 3,4-disubstituted aniline as accelerators for curing unsaturated polyester resins used as filling compounds. The aniline is substituted with two radicals R.sup.3 and R.sup.4, one of which represents C1-C4 alkyl radical or a C5-C6 cycloalkyl radical and the other of which represents a halogen atom. The halogenated aniline accelerators are stated to provide good sandability, long shelf life and prevent discoloration of polyester resins.